1. Technical Field
This invention relates generally to apparatus adapted for subsurface disposal, and, in particular, to source and sensor assemblies for such apparatus.
2. Description of Related Art
Exploration, production, and monitoring of hydrocarbon and water deposits entails the measurement of subsurface characteristics and the evaluation of the obtained data to determine petrophysical properties of interest for the relevant formation or reservoir. These techniques typically employ logging instruments consisting of an elongated tubular or “mandrel” equipped with sources adapted to emit energy through a borehole traversing the subsurface formation. The emitted energy interacts with the borehole fluid (“mud”) and/or the surrounding formation to produce signals that are detected and measured by one or more sensors on the instrument. By processing the detected signal data, a profile or “log” of the formation properties is obtained.
Logging techniques known in the art employ various forms of conveyance for respectively deploying and retrieving the instruments to/from subsurface formations. Among the various logging techniques, two forms of conveyance are routinely employed—wireline and while-drilling logging systems. Wireline logging entails lowering the instrument into the borehole at the end of an armored electrical cable to obtain the subsurface measurements as the instrument is moved along the borehole. Logging-While-Drilling entails disposing the instrument in a drilling assembly for use while a borehole is drilled through the formations.
Conventional electromagnetic (EM) logging instruments are implemented with antennas that are operable as sources and/or sensors. In operation, a transmitter antenna is energized by an alternating current to emit EM energy which interacts with the formation and a corresponding EM signal is received at a receiver antenna. U.S. Pat. No. 6,788,263 describes systems for EM logging. Some logging instruments are implemented with acoustic sources and sensors to obtain the measurements using acoustic energy. U.S. Patent Publication Nos. 20050150655, 20050150713 and 20050152219 describe systems for acoustic logging. Other logging instruments are implemented with sensors configured for spherically focused log (SFL) measurements. U.S. Pat. No. 7,042,225 describes systems for SFL logging.
FIG. 1 shows a conventional sensor 10 mounted on a tubular 12, such as the mandrel of a wireline instrument. The sensor 10 includes an exposed electrode 16 that comes into contact with the borehole fluid when the instrument is deployed in a borehole. FIG. 2 shows a cross-sectional view of the sensor 10 configuration of FIG. 1. As shown, the electrode 16 is disposed over an insulation layer 18 and held in place by an outer layer 20, which may be formed from a composite material. In this design, the connecting wire 22 from the electrode 16 has to go through the insulation layer 18 in order to make the electrical connection with an internal component 24. Due to the space constraints typically imposed on the instruments, the thickness of the insulation layer 18 is often smaller than desired for forming a good seal for the wire 22 under downhole pressure. The slim tubular 12 designs also have the problem of bending and poor eccentricity that affects the actual thickness of the layer 18. These drawbacks of conventional designs are among the causes for failures of the sensor 10 under the pressures experienced when the instrument is disposed downhole.
As logging systems continue to evolve with highly integrated tools, the sources and sensors are often exposed to downhole fluids, and wires from these components have to be connected to internally housed electronics that cannot be wet or contaminated by the downhole fluids. Thus a need remains for improved techniques to implement tools with sources and sensors for subsurface use.